Many semiconductor devices are used in a packaged form in which a semiconductor element mounted on a circuit substrate is sealed with resin.
As a method for packaging a semiconductor element, there has generally been used a method in which a lead-frame (a sheet metal, a TAB tape or the like) with a device mounted thereon is fixed in a mold, and a melted resin is injected to the mold and solidified therein to seal. However, this method has a limitation in manufacturing a thin-walled package. This is because a mold with high precision in the thickness is difficult to make, and even though such a mold can be produced, the fine structures (circuits, wires and the like) of a device may be damaged by flow pressure of the resin that is injected into a narrow space. Further, with resin sealing using a mold, small-lot production and mass-production require molds with comparable quality, so that the production cost of the mold is concerned about in small-lot production.
For these reasons, there has been proposed a method for sealing a semiconductor element with a resin sheet in place of a mold.
For example, Patent Document 1 discloses “a method for manufacturing a semiconductor package in which a semiconductor element mounted on a circuit substrate having a wiring pattern and the connecting part of the wiring pattern and the semiconductor element are sealed with resin, wherein a resin is applied in an amount in accordance with the size of the semiconductor element on the surface of a base tape to form a pattern in accordance with the shape of the semiconductor element; the resin is melted in accordance with the mechanical strength of the semiconductor element and the connecting part; the semiconductor element and the melted resin are aligned by adjusting the positions of the circuit substrate and the base tape; a given pressure is applied to the aligned resin and the semiconductor element to embed the semiconductor element into the melted resin, thereby sealing the semiconductor element and the connecting part without being damaged”. Patent Document 1 described an uncured thermosetting epoxy resin as the resin used for sealing.
However, the following problems have been found in sealing with such an uncured epoxy resin as described above.    (1) In a mounting method as referred to a flip-chip mounting method, a chip and a circuit substrate are connected through solder balls formed on the undersurface of the chip. In this mounting method, the side surface of the chip is at almost right angle with the circuit substrate plane, that is, the chip is protruded almost perpendicularly out of the circuit substrate plane. Even though a semiconductor element is embedded in melted resin by the method of Patent Document 1 above, the rectangular part (around the legs of the chip) is not fully filled with the resin and voids are formed. To fill the resin sufficiently, the semiconductor element may be embedded in the resin at high pressure. However, high pressure is difficult to apply to the melted resin in a fluid state, and the semiconductor element may be damaged if pressure is excessively applied.    (2) In a mounting method as referred to a wire-bonding mounting method, an IC chip and a circuit substrate are connected with extremely thin gold wires. The gold wires may be deformed or broken even by slight external force. In the aforementioned method of Patent Document 1, it is described that a semiconductor element is embedded in a melted resin, sealing the semiconductor element and the connecting part without being damaged. However, with the uncured epoxy resin specifically disclosed in Patent Document 1, if used alone, the melt properties thereof are difficult to control to be within a proper range. For example, when the fluidity of the melted resin becomes too high, the resin also diffuses into other parts beyond parts where necessary, causing fouling or defective appearance of the device. On the other hand, when the fluidity of the melted resin is too low, the gold wire may be deformed or broken before embedded in the resin.Patent Document 1: Japanese Patent Laid-Open Publication No. 251347/1999.